Fueled by the increased popularity of more capable devices (such as smart-phones, tablets, etc.), mobile broadband is experiencing an exponential growth. To effectively handle the data explosion challenge, Heterogeneous Network (HetNet) deployments have emerged as the key network evolution path. In such deployment paradigm, low-power small cells are envisaged to boost capacity at hotspot areas, whilst wide area connectivity is provided by the macro overlay. Particularly, small cell operation at license-exempt frequency bands has gained significant momentum during the last years. This is driven by the sparse availability of new licensed spectrum, making IEEE 802.11 Wireless Local Area Networks (WLAN) a cost-effective solution for complementing 3GPP 3G/4G networks. In this context, the existing massively deployed WLAN infrastructure and the high WLAN penetration of User Equipment (UE) devices provide excellent opportunities for offloading cellular traffic to WLAN. Current devices commonly support a rather simplistic offloading scheme by blindly attaching to WLAN, whenever possible. However, such mechanisms often result in noticeable throughput degradation after switching to WLAN. The reason is due to the Medium Access Control (MAC) nature of WLAN systems where network nodes (in downlink) and users (in uplink) have to compete for occupying the transmission channel. This essentially calls for the design of smart offloading schemes that opportunistically exploit WLAN without jeopardizing user experience.
Two new paradigms have appeared in recent years to advance the performance of these HetNets, namely: Link Aggregation (LA) and Dual Connectivity (DC). These exploit the fact that the user device supports two (or more) radios which can operate at the same time. For example, the user device could operate two Long-Term Evolution (LTE) radios (on different frequencies) at the same time, or LTE and WiFi.
In link aggregation, a stream of data packets can be split and delivered over two links operating simultaneously. The aggregate throughput is the sum of the throughput on each link.
In dual connectivity, there is an anchor node (for example, an LTE Evolved Node B (eNB)) that provides wide area coverage and signaling connectivity, whilst subtended small cells provide high bandwidth user plane links to users. Small cells of different Radio Access Technologies (RATs) and using different spectrum (including unlicensed spectrum) may be attached to the anchor node.
In 3GPP Release 12 (R12) and currently in Release 13 (R13), different realizations of these concepts have or are being standardized. In R12 LTE Dual Connectivity was introduced and in R13 there are work items to standardize (i) LTE/WLAN Aggregation (LWA) and (ii) License Assisted Access (LAA) to aggregate licensed and unlicensed LTE carriers.